Surge protectors are used in a wide variety of commercial and consumer applications to protect AC power distribution systems and various types of electrical and electronic equipment that receive operating power from such systems. AC power distribution systems and the electrical loads that receive power from such systems can be subjected, often repeatedly, to a potentially damaging and/or hazardous condition commonly referred to as a “power surge.” A power surge is characterized by a sudden large transient increase in the magnitude of voltage one would not normally expect to encounter at a given point in the AC power distribution system. Typically caused by lightning strikes or equipment failures of some kind, power surges can damage or destroy electrical insulation systems as well as motors, switches control devices, computer systems, and all other types of electrical or electronic equipment. They may result in fire and/or electrical shock hazards capable of causing death, serious injury, and/or ancillary property damage extending beyond the limits of the electrical system or equipment subjected to the power surge itself.
In order to prevent or mitigate the effects of power surges, various types of TVSPD units are known in the prior art. TVSPD units commonly include any of various kinds of devices which, at voltages within normal ranges of the nominal operating voltages of the AC electrical system or device they are intended to protect, exhibit relatively high electrical impedance, but which, upon being subjected to a voltage of sufficiently high magnitude, exhibits a significantly lower impedance and conduct electrical current relatively readily. Non-limiting examples of various types of SPDs include varistors, silicon avalanche diodes, zener diodes, selenium cells, gas discharge tubes, and high voltage capacitors, of which the metal oxide varistor (MOV) is favored for many low voltage applications, such as in AC power distribution systems having normal, nominal operating voltages of about six hundred volts AC (600 VAC) or less and medium voltage applications which typically operate at up to about four thousand one hundred sixty volts AC (4,160 VAC). Although they are referred to by various names, such devices are often called “surge protection devices” (SPDs). As used herein and in the appended claims, the term “surge protection device” or “SPD” is to be construed broadly to refer to any type of device that exhibits suitable voltage-dependent impedance characteristics, regardless of whether the device is of a presently known type or one that is developed in the future.
In addition to SPDs, surge protectors optionally include one or more thermal or overcurrent protectors. As used herein and in the appended claims, the term “overcurrent protector” refers to a fuse, circuit breaker or any presently known or future developed device for interrupting or limiting the flow of an abnormally large electrical current. To be similarly broadly construed, “thermal protector” is to be construed broadly to refer to any type of device which, in normal operation, exhibits a relatively low electrical impedance, but which, upon being subjected to temperatures above a threshold value, opens the circuit entirely or exhibits a significantly higher impedance and substantially prevents the flow of electrical current. Non-limiting examples of various types of thermal protectors include thermal fuses, bimetal thermostats, thermal cutoffs, thermal cutouts, and thermal links.
A surge protector which includes SPDs as well as overcurrent and thermal protectors is disclosed in U.S. Pat. No. 6,122,157 to Gerlach, which is expressly incorporated herein by reference in its entirety to form a part hereof.
Transient overvoltage protectors of the prior art readily accommodate transient voltages of varying magnitude but are quite limited with respect to the range of voltages at which they can operate properly for sustained periods under normal conditions. Relatively small deviations or fluctuations of normal operating voltages from a specified rated or “nominal” input voltage are expected. These have generally not been a problem. However, in order to accommodate a change from one normal operating voltage to a significantly different normal operating voltage, it has typically been necessary to remove and replace a previously installed TVSPD unit with one having a normal operating voltage rating which corresponds to the new normal operating voltage. Doing so, entails not only the expense of purchasing a new surge protector, but also the time, lost production, labor expense necessary to remove the old device and install a different one.
One scenario which has required the removal and replacement of surge protectors is that in which one electrical load device is physically removed and substituted for another load having a need for a different nominal input voltage than the load device that was removed. For example, replacing a 1,000 volt AC motor with a new 4,160 volt AC motor would also require complete replacement of the TVSPD. An example of an application where this routinely occurs is that of motors used to drive pumps at wellheads in the oil and gas industry. Motor horsepower requirements can vary dramatically from one well to the next and often change over time at a given well as conditions change. Pumping material from increasingly greater depths and/or pumping material of greater viscosity imposes greater demand on the motor. The mechanical load on the motor can also increase as the pump, and the bearings in the motor itself wear over time.
It is a common practice to compensate for such changing conditions by replacing a motor with one having a different nominal operating voltage when operating conditions change. It is not at all uncommon for a drilling operation to use a variety of different motors whose nominal input voltage requirements may span a range of four-to-one (4:1) or more. That is, a given motor may operate at a nominal input voltage that may be more than double, or even more than quadruple the voltage used to drive a different motor. A prior art surge protector suitable for protecting the pump motor at a nominal 1,000 VAC input voltage is unsuitable if the motor input voltage is a nominal value of 2,400 VAC. The TVSPD used initially must be removed and replaced entirely when such a change is made. If the motor voltage is later changed again, for example, to one having a nominal input voltage of 4,160 VAC, the second TVSPD must again be removed and replaced with yet a third one, and so on. The time, effort and monetary cost of carrying out such replacement of TVSPDs has been substantial. The prior art has not provided a surge protection apparatus which overcomes those drawbacks and can provide effective transient voltage surge at two or more nominal input voltages which may differ substantially from one another.
A TVSPD unit which has failed, cannot perform its intended function but the failure may not be known until a load device which would otherwise have been protected is destroyed by a lightning strike or other transient overvoltage condition. In the oil and gas industry and other applications, TVSPD units are frequently located in remote locations or ones which are otherwise difficult to access. In such cases, it has heretofore been a problem to monitor the operational status of the TVSPD in a simple, economical and reliable manner.